Ranma thinks of Ukyou as a guy. He's thought that for
over ten years - that way of thinking just won't suddenly change over
night. If Ranma was to marry Ukyou, HE'D have to be homo. He's clearly
not, and is too macho to even think that way.

How much does gender matter to a six year old?  Ranma thought of Ukyou as his
friend; knowing Ukyou's gender would make no difference in his thoughts about
her.
By the time Ranma got old enough to notice girls, it would have been years
since he had last seen Ukyou.

And, since he's always THOUGHT of Ukyou as a guy, he's not about to stop
just because he sees she's female. Would you marry a girl who used to be
a male, ten years ago?

Kun-chan...